3 rd Grade
Astronomy Lessons
The following is a set of suggested activities for a third grade curriculum unit on the Earth/Sun/Moon system. The goal is to provide students with an understanding of the motions of the three objects in the system and the way in which they determine the periodic changes we observe. In particular, students should develop an understanding of the way the Earth's daily rotation determines the cycle of light and dark that we call day and night; the way the Moon's motion about Earth determines the monthly cycle of lunar phases; and the way the Earth's orbital motion around the Sun determines the annual cycle of seasons. We also discuss how eclipses – solar as well as lunar – come about.
One important aspect of these notes is that we have made every effort to structure them so that students have an opportunity to observe in Nature as many of the phenomena under investigation as is possible. The challenge here is that some of these, like seasonal changes, can only be observed by making observations in different seasons – requiring that the
teaching of this unit be distributed over essentially the entire school year. The activities described here are an attempt to combine the temporal requirements of observation with a sound pedagogical development of the material. A suggested timeline below demonstrates how these activities can be timed for optimal success.
Additional materials, still under development, include a unit on light, shadows, and images.
Some early versions of some parts of this can be found at
http://www.cgtp.duke.edu/~plesser/outreachstuff/ . Because light and shadow play such an important role in understanding the phenomena covered here, we recommend in fact that the light unit be taught before bulk of the Earth/Sun/Moon unit, as reflected in the timeline.
These cyclic changes are of fundamental importance to our lives, and their relation to
celestial goings-on has been the subject of intense scrutiny – scientific, religious, and literary – in all cultures since the dawn of civilization. The literary products of our fascination with celestial motions and their meaning form an ideal literacy connection to this science unit. In an appendix we have compiled some stories from various traditions that we found
particularly enriching. These are at best representative, certainly not exhaustive.
In teaching students these subjects it is natural that many questions will arise that are not
addressed here, but would be natural extensions of the material included here. Examples of
this are the nature and structure of the Sun and the Moon, their history and origin, the story
of human exploration of space in general and the Moon in particular, etc. We have included
some fragments of such information in the Teacher Background sections of the units but
teachers should expect many relevant questions not answered here to come up.
The curriculum materials presented here were produced by J. Heffernan and R. Plesser.
They are based on material developed by Ronen and presented at Forest View Elementary in Durham, NC by Ronen and by undegraduate students from Duke University, and were
written up in their current form by John in the summer of 2003, with support from the NASA Space Grant program.
Finally, we have found that teaching this unit has been significantly enhanced by field trips to the Duke Observatory to observe some of the concepts in the sky. If observatory visits are not practical, we strongly recommend an evening meeting for naked-eye observing to render abstract concepts concrete.
Light/Astronomy unit for Third Grade (Wishful) Timeline – 2004/5.
8/13 school starts
Draw season of your birthday; arrange pictures around classroom walls creating a one-year cyclic calendar around the class. Use this for Math tie-ins!!
Can create a connection to the cycle of hours in a day, develop a feel for what 6 pm or 3 am means. Talk about 24 hours is a day, 12 hours is half of that (day or night).
Do a Sun path measurement. Note sun should rise and set slightly North of East/West (exactly East/West at equinox).
9/7 Start 4-5 weeks of light and shadow stuff with Duke volunteers.
9/21? Equinox – provide some context.
Keep track of Sunrise/Sunset times (from papers, web) for a week as part of morning activities; keep the data for future use!
4-5 weeks of light lessons Day and Night lesson Reason for the Seasons.
Phases of the Moon
Eclipses
10/27 total lunar eclipse
Convert wall calendar into Earth orbit, referring to previous discussions.
12/10 Do a Sun path measurement. Note Sun should rise and set South of East/West.
Shadow of Time
Keep track of Sunrise/Sunset times (from papers, web) for a week as part of morning activies; keep the data for future use!
12/20 Solstice – can discuss meaning of this in more detail.
Keep track of Sunrise/Sunset times (from papers, web) for a week as part of morning activies; compare the three sets of data from three seasons.
3/21 equinox – final summary of seasons, orbits, etc.
List of Activities
1.
The Sun Moves in the Sky: Examination of the Sun’s movement through the sky.
2.
Shadows of Time: Using a stick’s shadow to examine the Sun’s movement and the passage of time.
3.
Day and Night on the Spinning Globe: Exploration of the how and why of darkness and light.
4.
Reason for the Season: Discover the causes behind our seasonal changes.
5.
Phases of the Moon: Each student will create the phases of the moon.
6.
Where did the Moon Go? The story behind eclipses and who’s in the way of whom.
Daily Classroom Routines
One of the most powerful and productive tools to help students internalize the rhythm of the cyclic processes, as well as the intricate three-dimensional geometry that governs them, uses daily classroom routines that track the various changes. Depending on taste, available time, etc. individual teachers can pick and choose among the following recommended activities.
1) Chart sunrise and sunset times. These are available in local newspapers or online.
Keeping the charts allows students to keep track of the changing length of day. This need not be done throughout the year, a sample of a few days near fall equinox, a few days near winter solstice, and a few near spring equinox should be sufficient to
demonstrate the pattern.
2) Record daily high and low temperatures. Keeping track of these helps monitor the seasonal changes.
3) Record Moon phases, moonrise and moonset times. The periodic changes in these, and the correlation between them, help demonstrate one of the trickier aspects of
understanding how phases occur.
4) Construct an Earth calendar in the classroom. This exercise is very useful in helping students visualize the three-dimensional geometry involved in daily, monthly, and annual cycles. Imagine that the Sun is located near the center of the room. Some teachers have realized this by hanging a papier-mache Sun from the ceiling at the appropriate point. A globe, mounted so that it can be moved around the room along the walls, will represent the Earth moving in its orbit about the Sun. One wall will roughly correspond to each of the four seasons (there is usually one wall – with
cubbies, door, etc – along which the globe should not be positioned, and selecting this to correspond to summer is a good idea). To begin with, therefore, you will need to mark off along the walls locations corresponding to dates in the year. The Earth moves about 1º along its orbit every day (more precisely it moves 1/365 of a 360º circle per day), so marking off every 7º along the wall provides one mark a week. The globe should be mounted with its axis tilted so the North pole points in the direction of the wall corresponding to winter, and kept that way as it is moved. The daily, or weekly, activity of moving the globe along the orbit helps students recognize the relation between the Earth's orbital motion and seasonal changes. It is also helpful to mark the classroom's location on the globe and rotate the globe to a position corresponding to the correct time of day.
The Earth calendar can be extended and enhanced in various ways. Students can
decorate the walls with pictures depicting the corresponding seasons; each student can
produce a picture depicting the season in which his or her birthday occurs, and these
can be placed along the walls in the appropriate place. Also, to demonstrate how the
night sky changes over the course of the year, models of the constellations of the
Zodiac can be positioned on the walls in locations corresponding to their position in
the sky (constellations of the Zodiac are located in space near the plane of the Earth's orbit and the time of the year to which each is associated by astrologers the Sun and the relevant constellation are approximately aligned as seen from Earth). From any
position along the orbit, the stars that are visible will be those on the “night” side of Earth, away from the Sun.
When learning about lunar phases the model Earth can be endowed with a model Moon (a white ball, smaller than the globe) mounted so it can be rotated about the globe. The daily activity can now be extended to include setting the Moon in its correct position relative to Sun and Earth as determined from its phase.
Many other extensions can be created, these are a few that have been successfully applied at Forest View.
Glossary
Enchanted Learning’s Astronomy Glossary located at:
http://www.enchantedlearning.com/subjects/astronomy/glossary/index.shtml
Teacher's Background
Setting and Scales
The cycles of day and night, seasons, and lunar phases are all governed by the relative
motions of Earth, Sun and Moon in space. The Earth is a roughly spherical object, of radius about 6400 km. Its shape, which crucially affects all the phenomena we study, was first deduced by Aristotle from the fact that Earth's shadow on the Moon appears rounded
whenever it is visible (during a lunar eclipse, see below) and the geomtric fact that a sphere is the only shape that projects a round shadow when illuminated from any direction.
Eratosthenes in fact made a roughly correct measurement of the radius around 400 BC, though this knowledge was forgotten by Europeans for centuries.
Surrounding the Earth is a thin layer, about 150km thick, of air, the atmosphere. The fact that we live our lives within this envelope makes thinking about the emptiness that
comprises the great majority of the Universe a bit confusing. On Earth, light from the Sun or
from any other object scatters off objects around us or off impurities in the air. Thus, we are
bathed in light from all directions. In the emptiness of space, with nothing to scatter it, light
from the Sun, for example, streams away from the star in straight lines. An astronaut in
space looking at the Sun would be blinded by its brightness, yet the sky near the Sun would
appear black except for the pinpoints of distant stars. Images taken during the lunar day, in
which the surface and objects on it appear brightly illuminated by the Sun, yet the sky is
dark, are a powerful example of this – the Moon is too small to bind an atmosphere.
Light in the vicinity of Earth arises mostly from the Sun, a rather average star with a radius of some 690,000 km and a surface temperature of 5800K. The interior of the Sun is heated by nuclear fusion (the same process that powers Hydrogen bombs) to much higher
temperatures of some 1.5 million K, and the energy produced by fusion is radiated from the surface as light and heat. Some 150 million km from Earth, the Sun is by far the brightest object in the sky because it is the nearest star to us by far. The next nearest star is 300,000 times farther.
One of the challenges of learning and teaching about space is that distances, sizes, and times involved are so large as to defy intuition. To gain some sense of scale, it is helpful to
consider a scale model of the Solar system in which the Earth is represented by a ball of radius 6.4cm (about the size of a grapefruit). At this scale, the Sun – 10000 larger in radius – would be represented by a ball of radius 700m (about half a mile) at a distance of about 15km (10 miles). The next nearest star would then be 3 million miles away, ten times farther than the distance to the (real) Moon.
In thinking about space students are also often confused by common sense assumptions acquired in their life on Earth's surface. On Earth, we are surrounded by the atmosphere, while most of space is to a good approximation an empty vacuum. We are all aware that this makes breathing in space impossible, but the atmosphere affects our experience in other ways as well. The one most pertinent here is the fact that light – from the Sun or artificial sources – is scattered by impurities in the atmosphere: dust, water, ice, etc. This is the reason our daytime sky is a luminous blue (see the light unit notes). Typically on Earth, we are also surrounded by other objects. These reflect light, with the result that we are “bathed”
in light from all directions. In the emptiness of space, with nothing to scatter or reflect it, a beam of light will propagate in a straight line for great distances: this is what enables us to see distant stars or galaxies. This also means space is dark, even in the vicinity of a bright object like the Sun. Any object reflecting the light will appear luminous against the
perfectly black backdrop of space. This is why planets, and the Moon, appear to shine brightly in the night sky.
Another common misconception is that motion requires propulsion. This is a natural conclusion to draw from our experience on Earth's surface, where friction and gravity dominate. Nothing moves unless we move it, and left to their own devices objects quickly come to rest on the ground. In fact, a moving object upon which no forces act continues to move at a constant speed. Objects slow to a standstill on Earth due to the forces created as they move through surrounding air, or over the ground. In space, absent air or ground, perpetual motion is the natural state of things.
Gravity, Orbits, Motions, and Origins of the Solar System
The motions – and shape and structure – of astronomical objects are primarily determined by the action of the force of gravity. This is the universal attractive force that any object applies to any other object in the Universe. The gravitational force between two objects is
proportional to the product of their masses, and inversely proportional to the square of the
distance between them. Thus, the force applied by the Earth to various objects on its surface, which we call weight, grows larger the more massive the object in question. The force applied to those same objects by an ant, while nonzero, is too small to be measured because the mass of an ant is so small. Similarly, the force applied to objects by a distant galaxy is negligible despite the galaxy's great mass due to its immense distance.
The force of gravity causes an unsupported object to fall to the Earth. Yet the Moon,
unsupported, has avoided this fate for billions of years. Moreover, orbiting satellites do not fall, though they lack any jets or other means of propulsion (this is often misunderstood).
While it is true that gravitational forces weaken with distance, this is not the reason. The Moon, spaceships, and satellites, avoid falling to Earth because they are orbiting it. In essence, all these objects are falling to Earth but since they are also moving around Earth they manage to “fall” while maintaining a constant distance from the planet. One can
explain the motion of an object in orbit as a continual fall in which the Earth (or whatever is being orbited) is forever being “missed” and “overshot.” In this way the Moon is forever falling to Earth as it orbits, while Earth and Moon together are falling to the Sun as they orbit it.
Under the influence of gravity, a primordial nebula, a cloud of gas and dust, collapsed
inward upon itself to form what we now call the Solar system some 4.5 billion years ago. As the nebula collapsed, two important processes occurred. Like an ice skater pulling in her arms to initiate a twirling spin, the slight, random rotational motion of the nebula accelerated and became a pronounced overall rotation. As the rotation accelerated, the nebula flattened out into a disk. The rotation as well as the orbital motion of most objects in the Solar system reflect this original rotation. For this reason all the planets orbit the Sun in approximately the same plane and in the same direction; the Sun itself, as well as most of the planets, revolve about themselves in approximately the same plane and in the same direction; the Moon orbits Earth in approximately the same plane, etc. For example, this is the reason that all planets, the Moon, and the Sun show up in the sky on or near an imaginary circle – the shape of this plane from the point of view of someone like us who is inside it – called the ecliptic.
As the gas and dust became more dense, collisions between particles heated it up. Most of the matter in the nebula ended up forming the star at its center – the Sun. This densest part of the nebula was also the hottest, and eventually at its center temperature and density were sufficient to initiate the process of nuclear fusion. Material left over from the formation of the Sun continued to orbit the nascent star, and this eventually coalesced – driven once more by gravitational attraction – into the planets. The condensation of gas and dust into a dense planet heated the material to melting. Initially the Solar system contained hundreds of small
“planetesimals.” Some of these merged to form the planets, others were ejected from the
system by collisions, so that by 3 billion years ago interplanetary space had become almost
empty. Our leading theory of the Moon's origin involves a collision between the Earth and a
Mars-sized object during this early phase, in which parts of both colliding objects were
ejected into orbit where they eventually coalesced into our Moon.
Like most large celestial objects, the Earth, Sun, and Moon are roughly spherical in shape.
The reason, in the case of the Sun, is that the star is made mostly of gases (Hydrogen and Helium principally) held together by the force of gravity. Deviations from a spherical shape would be erased by gravity. Imagine attempting to raise a mountain of water in the middle of the ocean. It would be destroyed by gravity. On a larger scale, this is the mechanism that maintains the Sun's shape. In the case of the Earth, Moon, and planets, their shape is an indication that at one point they, like the Sun, were fluid in composition. Indeed the primordial Earth was a much hotter body than it is today and was mostly molten; it was at this time that it acquired its nearly spherical shape. Smaller asteroids, which never became hot enough to melt as they formed, have a variety of shapes and are not in general spherical.
Deviations from a perfect sphere are also instructive. Rotating bodies are usually oblate – thicker about their equator than a perfect sphere would be. This is certainly true of the Earth and of the Sun. Extreme cases of this occur in larger bodies such as the Solar system as a whole or our Milky Way galaxy, which have been flattened to disks by their rotational motion.
Another important deviation is due to the fact that gravitational forces on a large object are not uniform. For example, the Moon's mass applies a gravitational force to all objects on Earth. This force is strongest at the point on Earth nearest to the Moon, and weakest at the point farthest from the Moon. The average attraction, which causes a small orbital motion of the Earth as a whole, has little effect on us, but these differences – tidal forces – which tend to draw Earth out into an elongated shape with the axis pointing at the Moon, have a
profound effect. The Earth is too rigid to be deformed much, but water can flow on its surface, and the bulges on two opposing sides of Earth are locations of the twice-daily high tides. The tidal forces on the Moon due to the variation in the strength of Earth's attraction caused the early, fluid Moon to be slightly deformed. The Moon is indeed somewhat
oblong, and rotates as it orbits Earth so that its longer axis always points towards the planet, much like a visitor walking around a museum exhibit, keeping her face towards it as she moves. This is the reason we see the same side of the Moon at all times.
The Sun is the main source of energy in the Solar system. Energy emitted by the Sun as light and heat is the source of heat and light on Earth as well as the reason the Moon and planets are visible. Unlike stars, these cooler objects do not produce energy and do not emit light.
We see them in the dark night sky because they reflect Sunlight. For the Earth/Sun/Moon system, this means that the relative positions and orientations of the three objects drive a set of cyclic changes as perceived from Earth. These changes are the essential core of these lessons. The changes in relative positions and orientations occur on three different timescales: a 24-hour cycle governed by the Earth's rotation on its axis; a monthly cycle governed by the Moon's orbital motion about the Earth; and an annual cycle governed by the Earth's orbital motion about the Sun. Because of the different timescales involved, we can treat each motion ignoring the slower ones. This means that for the purpose of
understanding daily changes the motion of the Earth about the Sun, and of the Moon about
the Earth, in the course of one day can be ignored. For the purpose of discussing the Moon's
orbit about Earth we can at first ignore the fact that Earth moves about the Sun: it moves little in a month.
Daily Cycle: Rotation of the Earth
The fastest cycle is the daily rotation of Earth about its axis from West to East. The main result of this is that to an observer on Earth all objects in space seem to be moving in circles from East to West. This is helpfully thought of as the same effect that makes the scenery seem to whirl around an observer who is riding a merry-go-round. As the Earth rotates, objects in a given direction in space are revealed above the Eastern horizon (more precisely the horizon drops below them), arc through the sky, and are eventually obscured by the rising Western horizon (set). Note that this is true for moderate latitudes. At the poles, objects neither rise nor set as the Earth rotates, but move in horizontal circles in the sky. A given star (direction in space) is either always visible or always below the horizon at the pole. At any latitude, the sky moves as though hinged on an axis parallel to Earth's axis. At the equator this is horizontal, at the poles vertical. At intermediate latitudes the axis is angled to the horizon. Of course, the axis is fixed, and its differing orientations relative to the horizon reflect the curvature of Earth. What you consider vertical depends on where you are on Earth.
Stabilized like a spinning top, the axis about which the Earth spins is fixed. This means if you stand at the North pole and look straight up you will see the same part of the sky (the vicinity of Polaris, the Pole Star, at all times. Our discussion of the history of the Solar System above would suggest that the Earth's rotation and its orbital motion about the Sun be in the same sense, as both are descended from the primordial rotation of the Solar nebula.
This is approximately true. In fact the axis about which Earth revolves is tilted by about 23.5º from the axis about which it orbits. The same is true, to varying degrees, of all the planets in the Solar system, reaching an extreme case with Uranus, whose axis lies almost in the plane of its orbit. These deviations from our expectations likely reflect violent collisions early in the history of the system.
Most dramatic, of course, is the rising and setting of the Sun, which determines the daily cycle of light and dark at every point on Earth. Because this reflects the rotation of a round planet, different places on Earth face the Sun at difrerent times during the rotation. We all set our clocks so that the Sun will be at its highest point in the sky around noon, hence the need for time zones around the Earth. Another effect of this rotation, mentioned above, is that as different parts of Earth face the Moon, water on the surface moves in response to tidal forces leading to twice-daily tides. Because of the Earth's rotation, in fact, the high-tide axis is dragged “ahead” of the Moon's position – tides are highest at the points that faced toward the Moon or away from it several hours earlier.
An interesting fact is that the axis about which Earth rotates is not, in fact, completely fixed.
Like a top tilted relative to the vertical, the Earth “wobbles” slightly, so that its axis
maintains the 23.5º tilt but “precesses” in a cone about the orbital axis. This means Polaris
has not always, nor always will be, our “pole star.” Ancient Egyptian remains show that the
star Vega was the pole star 5000 years ago. The wobble is quite slow, however, the rotational axis completes an entire circuit about the orbital axis every 26,000 years or so.
Polaris is thus going to remain our Pole Star for quite a while. The slow change is referred to for historical reasons as “the precession of the equinoxes.”
Annual Cycle: Earth's Orbital Motion
The orbital motion of Earth around the Sun, which takes about 365 days and comprises a year, has several effects as well. Most obviously, it means that over the course of the year the direction from Earth to the Sun changes. This means that when we look up at night – which means we are looking away from the Sun – we see different stars in each season. This can be made real for students by posting pictures of the constellations on classroom walls.
The constellations visible in the spring would be on the wall near which the globe would be in spring. In the fall they would lie behind the Sun as viewed from Earth and not be visible at night. Stars located on the ceiling (near the North star) would be visible in all seasons.
A less obvious effect of the orbital motion but by far the most important one for us, is related to the fact that the Earth's axis (imaginary line running from South to North pole, about which Earth rotates) is tilted relative to the plane in which the Earth orbits the Sun. As the Earth orbits the axis points in the same direction at all times (this direction is the direction to the North star which is why that star is always found North). To understand the effect of this tilt is easiest if we consider two extreme cases, neither of which is in fact true.
Consider first what would happen if Earth's axis did not tilt at all. In the classroom model, this would mean mounting the globe with its axis vertical. Note that at any point along the orbit one half of the Earth's surface is illuminated by the Sun and one half is in the shadow of Earth. As the Earth rotates, each point on the surface (except the poles) moves in a horizontal circle so spends one-half of its time in the illuminated side and one-half in the dark. Sunlight thus lasts 12 hours everywhere throughout the year. Seasonal variation is completely absent.
On the other hand, consider what would happen if Earth's axis were in the plane of the orbit (horizontal in our model). (This is in fact the case for Uranus.) Remember that the axis points in the same direction throughout the orbit. The circles along which points move with the Earth's rotation are now vertical. Thus, at one point in the orbit the North pole would face the Sun. At the time of year corresponding to this point in the orbit, the Sun would never set in the Northern hemisphere and would never rise in the Southern hemisphere! At the opposing point on the orbit the roles of the two hemispheres would be reversed.
Seasonal variations would be very extreme. When the Northern hemisphere faced the Sun,
and all points North of the equator enjoyed continuous Sunlight, the weather in the Northern
hemisphere would be quite warm, while the Southern hemisphere, with no Sunlight at all,
would be cold. In fact, the region near the North pole would be warmest. This seems
intuitive, because it faces the Sun directly.
A more precise explanation is that near the point on Earth's surface nearest to the Sun, Sunlight will be impinging the Earth vertically, while closer to the equator sunlight is impinging at larger angles. This is most easily visualized by placing yourself in the Sun's position and looking towards Earth. You can see that from this point of view, features near this point seem larger than features just visible near the edge of the visible disk. The latter are “foreshortened by perspective,” and look smaller to you. Features that look larger to you – take up more of your range of vision – would correspondingly take up more of the incoming Sunlight. It is important to note that this has nothing to do with the actual distance to the Sun, is rather related to the curvature of the Earth and the angle at which Sunlight impinges on the surface.
In fact, the Earth is intermediate between these two extremes. Its axis, as is well known, is inclined by approximately 23.5º to the (perpendicular to) the plane of the orbit, leading to mild seasonal variations in climate. When the Northern hemisphere faces the Sun, it is warmer in Northern regions of the Earth – summer in the Northern hemisphere is winter in the Southern hemisphere. There is a common misconception that summers are warmer than winters because Earth is closer to the Sun in the summer. This is quite false, as
demonstrated by the fact that Northern summer coincides with Southern winter, but may be best corrected by noting that in fact the Earth is marginally nearer the Sun in January than at any other time of the year.
Monthly Cycle: Moon Orbits Earth
Intermediate in timescale between the daily rotation of the Earth and the annual orbital motion is the Moon's motion as it orbits Earth once every 29 days or so. The effect of this on the Moon's appearance to us is to create the familiar phases. The Moon is visible to us, as mentioned above, because reflecting Sunlight it appears luminous against the backdrop of empty space. The Moon does not, however, create its own light, so that we can see only that part of the Moon illuminated by the Sun. At any instant, the Sun illuminates precisely one half of the Moon's surface (just as it illuminates one half of Earth's surface). Similarly, at any given time we on Earth have an unobstructed line of sight to precisely one half of the Moon's surface. What we actually see of the Moon is that part which is simultaneously illuminated and visible.
When the Moon and Sun are in approximately same direction from Earth, the illuminated side of the Moon is the side facing away from Earth. The side facing us is dark and the Moon is invisible – a New Moon. Note that because this phase occurs when Moon is in same direction from Earth as the Sun, a new Moon rises and sets at the same time as the Sun.
Conversely, when the Sun and Moon are aligned in opposite directions from Earth, the
illuminated side of the Moon – the side facing the Sun – is also the side facing Earth and
visible. Hence we see an entire bright disk in the sky – a Full Moon. Because this phase
occurs when Sun and Moon are in opposite directions as seen from Earth, a full Moon
always rises at sunset and sets at sunrise. Intermediate phases occur between these points.
Because the Moon orbits Earth in the same sense as the sense in which Earth rotates – West to East – it rises later, by approximately 48 minutes, each day than it did the day before. The new Moon, rising at sunrise, is thus followed by a waxing crescent Moon which rises later and later as it waxes from day to day. When the Moon and Sun are 90º apart in the sky, one- half of the side of the Moon facing us is illuminated, and we see one-half of an illuminated disk in the sky. This is called waxing quarter Moon, and from what we have said a waxing quarter Moon is seen to rise around noon (and set around midnight). When more than half of the disk is seen, as the Moon continues to orbit away from the Sun, we have a waxing gibbous Moon rising in the afternoon and setting in the early hours of the morning.
The full Moon is followed by waning gibbous phases, with the Moon rising later and later at night, until the waning quarter Moon, when once more we see half a disk, which rises at midnight and sets at noon. This is followed by waning crescents, rising later and later in the early morning until the new Moon recurs.
The Moon does not, in fact, orbit Earth precisely in the plane of Earth's orbit of the Sun (the ecliptic). It orbit is in fact tilted slightly (5º approximately), and like the Earth's axis it
precesses. The result of all this is that the Moon is “above” (North of) the ecliptic for half of its orbit and “below” (South of) the ecliptic the other half, meeting the plane at two points located at opposite sides of the orbit (the nodes). When the orbit is oriented so that these points line up with the points on the orbit at which the Moon is new and full (we say “the line of nodes points at the Sun”) the new Moon can obstruct the Sun as seen from Earth, and Earth can obstruct the Sun as seen from the full Moon. This is when eclipses occur.
A Solar eclipse is possible due to the coincidence that the Moon, some 400 times smaller in diameter than the Sun, is also some 400 times closer to Earth. Thus both objects appear about the same size in our sky. When the full Moon intesects the ecliptic, it will cast a shadow on Earth. In fact, only in a small region on Earth (a few hundred miles across) will the alignment be perfect and the Sun totally obstructed. In this region, daytime will darken dramatically. With its luminous disk hidden from view, the outer layer of the Sun's
atmosphere – the corona – will be visible. Surrounding this small region – called the umbra – is a larger region in which the alignment is imperfect and the Sun partially obscured – the penumbra. In this region, the Sun's disk appears partially obscured. Sometimes, when a Solar eclipse occurs when the Moon is at a position in its elliptic orbit at which it is slightly farther from Earth than its average distance, so that it appears somewhat smaller in the sky, it fails to completely cover the Sun and a ring of the Solar surface is visible around the
darkened center, in an annular eclipse. Of course, during the hour or so that the eclipse lasts, the rotation of the Earth will have caused an extended swath of the surface to sweep through the umbra. Observers outside the penubmra will not observe anything unusual at all during a Solar eclipse. The new Moon will be invisible, and the Sun will shine as usual.
A lunar eclipse occurs when the full Moon is in the plane of the ecliptic, hence in the Earth's
shadow. As with a Solar eclipse, the more common penumbral lunar eclipse occurs when
the Earth only partially obscures the Sun (as seen from the Moon). Seen from Earth, the
Moon darkens slightly, often almost imperceptibly. More dramatic are umbral eclipses,
when the Earth obscures the Sun entirely (as seen from the Moon). Note that because the Earth is larger than the Moon, its umbra is large enough to cover the entire surface of the Moon and darken it completely. During a total lunar eclipse, the Moon darkens
dramatically, and reddens (the redness is caused by light from the Sun reaching the Moon
after traversing Earth's atmosphere, grazing the planet. As the Sun appears red at sunset
when its light traverses a large distance through the atmosphere before reaching our eyes, so
the light reaching the Moon is reddish, causing the Moon to appear red). Less dramatic
partial eclipses occur when the full Moon lies slightly above or below the ecliptic, so only
its lower, or upper, part is darkened.
The Sun Moves in the Sky
Purpose:
Our understanding of the cyclic changes all around us begins, historrically and conceptually, with observations. The most immediately accessible is the daily cycle of night and day, and more specifically the daily motion of the Sun across the sky from East to West. This is an important regularly repeating phenomenon, and formed the basis of ancient cosmologies, in which much effort was devoted to the question “what moves the Sun in the sky, and why, and where does it go when it sets.”
Our understanding of this phenomenon changed dramatically with the discovery that the Earth is round, and that sunset is a local phenomenon. The Sun, when it sets, does not disappear, it is shining elsewhere on Earth. Today, we think of the daily motion of the sky from East to West as reflecting the rotation of the Earth itself from West to East. It may be useful to point out that it is completely possible to think of the Earth as stationary, provided one considers not only the Sun but the entire Universe to revolve around us. This point of view is not popular among scientists and leads to a more complicated description of the motions of objects, but fundamentally it is equivalent to the standard approach, described as seen from Earth. Somewhat pompously, one can say that Einstein's theory of Relativity which shows that there cannot be a “correct” point of view from which to describe motion, that in fact all points of view are equally valid, tells us a cosmology in which the Earth is stationary is perhaps less simple but no less valid than one with a rotating Earth. This may be a comfort to some students.
The three dimensional motions of Earth and other objects are conceptually challenging, and we will build towards an understanding of them gradually. In this first activity we are going to collect observations, as the ancients did, without at first worrying about interpretation. In this first activity students will observe and record the Sun's apparent motion through the daytime sky. The most important property of this motion is its regular periodicity, repeating very reliably every 24 hours.
By repeating this activity three times during the year in three different seasons (fall,
winter and spring), the students' data will reveal the seasonal changes in the Sun’s
overhead path and when compared will ‘shed light’ onto how the Earth’s tilt on its axis
and orbital motion around the Sun create the seasons.
Materials:
- Compass
- 3’x 6’ sheet of paper for large horizon representation - Digital camera or panoramic camera
- Yellow construction paper for cut-out Suns
-
The Sun’s Path Across the Sky sheet generated by teacher
- Class list with students divided into groups of fours to be “Sun Trackers”
The Sun Path Chart
(Taken from Washington State University, Energy Program at http://cru.cahe.wsu.edu/CEPublications/eb1857e/eb1857e.html)
The sun path across the sky is a function of two things: the earth's daily spin and its annual orbit about the sun. On December 21 the sun path is at its lowest altitude and of shortest duration. (See
Figure 5.) Figure 5
Lowest winter sun path.
As summer approaches, the sun path gets higher and of longer duration until it peaks on June 21, the longest day of the year. The sun path attains its highest altitude at midday. (See Figure 6).
Figure 6
Highest summer sun path.
After June, the sun path gets lower and the days get shorter until the cycle is completed on December 21 and begins all over again.
By facing south it's possible to visualize the paths the sun follows on the 21st day of four months of the year and generate a graphic representation of the sun's various positions in the sky. (See Figure 7, page 5.)
Figure 7
Three specific sun paths.
Developmental Issues:
Eight and nine year olds are becoming more adept at holding and comparing two different objects in their mind when problem solving. The use of a physical Earth-Sun model is critical in the students’ attempts to visualize and remember the Earth-Sun system. Eight and nine year old students are becoming competent at collecting information and representing data pictorially in order to interpret what was observed and recorded. Lastly, cooperative teamwork is advancing to a point where academic problem solving can be aided by
providing specific roles for students within a team structure.
Inquiry Activity
Activity Preparation:
a) The day before the activity go out to the location where you will have the kids draw their Sun drawings and use a compass to locate the direction of magnetic South. Find a location that has an expansive view of the southern horizon from East to West, that is not shaded at any time of the day, and to which you can return every hour throughout the school day. Note that the Sun should appear due South around Noon but not exactly at Noon due to daylight savings, seasonal variations, and our longitude. Mark an arrow on the ground with chalk pointing South and be sure to have the kids facing South the next day.
b) Take a series of photos (digital, panoramic disposable, or 35mm will do fine) of the
horizon where the students will be drawing. Attempt to piece together your photos and print out a representation of the horizontal landscape that your students will use to help keep track of the Sun. Ideally, print out your landscape onto an 8”x11” piece of paper for each child to use as their “The Sun’s Path Across the Sky” sheet and also duplicate the outline of your landscape onto butcher block paper that’s about 3’ x 6’ for the entire class. The large horizon representation will be mounted in the class for your student to use as groups record their hourly observations.
c) If you are unable to make a photo representation draw a free hand representation of the horizon from your observation location. Be sure to make distinct landmarks on the
landscape. (e.g. trees, buildings, etc…). Create an 8”x11” horizon recording sheets for your students and a large 3’x6’ horizon representation to be mounted in your classroom.
d) Divide your class into groups of four students to be “Sun Trackers”
Activity:
1. Early in the school day, possibly in your morning meeting, ask your students, “Did
anyone see the Sun this morning as you were coming to school?” As you see hands rise
ask the follow up question of “When you were in the bus, in the car, or walking outside where in the sky would you describe the Sun was located? Close to the trees and building or higher up in the sky more straight overhead away from the trees and buildings?”
“Does anyone see the Sun in the morning close to the same place each day?” “Does the Sun stay in the place you saw it in the morning all day long?” “What is going on?”
2. Describe to the students that today they will be drawing the path or movement of the Sun in the school day sky. Noticing and drawing the Sun’s path has been done by scientists for thousands of years and has lead to many discoveries. Let’s see what discoveries your students’ drawing and discussions led to in this activity.
☼ Prior to going outside clearly explain that we will find out the Sun’s place in the sky without looking directly at it, not even for a glimpse. Describe the safety hazards of looking directly at the sun. ☼
3.
Explain the importance of drawing or recording what we notice today about the Sun’s path so that others can understand what we saw and discovered. Hand out copies of the
“The Sun’s Path Across the Sky” recording sheet you have drawn or photographed.
Describe how you drew or photographed the horizon and the importance of making the horizon representation accurate so their collected data or recordings are accurate.
4.
Have your students gather a pencil, clipboard, and their “The Sun’s Path Across the Sky”
recording sheet and travel to your outdoor observation location. Be sure to have your own clipboard, pencil and recording sheet. Once at your location, have everyone face the south. Explain the need to have the same viewpoint so we can draw and talk about the same thing. Brainstorm ways they can connect where the Sun is in the sky by noticing or feeling the Sun as it relates to other objects. Possible suggestions include: seeing
shadows, feeling warmth on the right or left side of your face, and noticing a landmark on the horizon. Explain to the students they will be following the Sun’s path along the
horizon in the South. Define the horizon as the place where the Sky and Earth meet.
Discuss the objects that are on the horizon especially the heights and shapes of different trees and buildings. Have them compare what they see in the actual horizon to what is on their representational “The Sun’s Path Across the Sky” recording sheet. Describe the importance of landmarks on the horizon in identifying where the Sun is in the sky. Teach your students how to measure the height of the Sun above the horizon using their fists.
Model how to place one fist vertical or thumb up on the horizon and continue to alternate fists, counting up until you reach the bottom of the Sun. Next draw the Sun in the sky where everyone agrees it to be. Record the time above the sun and return to the
classroom.
5.
Once in the class, discuss how the Sun they drew was a model. Explain how what we
observe and record outside will be duplicated or copied onto our larger 3’x6’ horizon
representation in the class. Explicitly show how the enlarged landscape model in class is
similar to the smaller “The Sun’s Path Across the Sky” sheet. Ask for a volunteer to place
a pre-cut out yellow construction paper Sun in the place where they recorded the Sun to be
in the sky. Be sure the student labels the recording time above the Sun and describes what landmarks he/she used to decide where to exactly place the Sun. Ask the class, “If you go out and draw the Sun’s position in one hour, how will its position be different from out first drawing?” “Why will it be different?” Have groups of students discuss their
predictions together and be prepared to share their thinking with the class. Invite one student from a group to come up and place a new pre-cut yellow construction paper Sun in the place where the Sun will be in one hour. Ask them to explain why they think the Sun will be in this location.
6.
In one hour, have the first group of “Sun Trackers” (a group of four students) return to the exact location outside to observe and record the Sun’s new location in the sky on their
“The Sun’s Path Across the Sky” sheet. Explain the importance of accuracy and using the landmarks to assist in locating the Sun’s location. Instruct the “Sun Trackers” that they will need to locate and draw the Sun’s position in connection to a landmark or object on the horizon and record the exact time of their observation. Once back in the classroom after their outdoor drawing, gain the class’ attention and ask the group that returned from outside, “How did the group’s predictions an hour ago match where the Sun is now?”
Next ask them, “Where do you think the Sun is going to be next hour and explain your thinking?” and have them place a new pre-cut out sun on the large sheet.
7. Have a new group of “Sun Trackers” go out each hour and make a drawing of the Sun’s location in the sky above the horizon. After each group returns follow the same steps as outlined above in #6. Each group observes, records, predicts and explains. After several drawings a definite shape is forming from the Sun’s path. Ask, “What type of shape is the Sun’s path making and how would you describe it?” Listen for what names the children give to describe the Sun’s arc. Additionally ask, “What shapes can be made from the Sun’s path? Half-circle? Circle?”
8.
Finally ask, “”What happens to the Sun at night?” Students will probably come up with several explanations, among them the correct one – that it is shining elsewhere on Earth's round surface. At this time it is not necessary to provide a “right” answer, and a serious discussion of the merits of whatever proposals they make is probably the most productive approach. The idea here is to present the problem, pique their curiosity, and start them thinking about it. Promise that the subject will be pursued further later on.
9.
After completing the entire day’s observation keep the large The Sun’s Path Across the Sky up for display in the class. If you have a South-facing window try to make the Mid- Day marks outlined below.
Class Noon-Line Project
Another project could be making a window Noon Line. This was a device used by farmers in Skåne, Sweden to mark Midday. Find a south-facing window, which has several panes of glass.
Notice how the solid framework between the panes casts shadows onto the window sill. When the Sun is at its highest point (at Midday) you can mark the line of the shadow cast by the pane frame with masking tape. This effectively turns the window into a simple sundial. Whenever the shadow of the pane frame matches the line of tape that means it is Midday.
Shadows of Time:
Using a stick’s shadow to examine the Sun’s movement and the passage of time
Purpose:
We will extend discovering connections between the path of the Sun across the sky and the passage of time using direct observation and recording. While the last activity helped solidify our understanding of the Earth’s shape and movement, this activity will continue to use the Sun’s path across the sky but this time so we can make accurate measurements of its path. The measurements will keep track of the stick’s shadow and allow us to read a sun clock. The sun clock will enable students to visually understand the relationship between the sun’s motion and our concept of time.
Optimally, you should do this activity three times during the year. Allowing your students to record the changes in the sun’s movement during three different seasons translates into understanding the tilt of the Earth on its axis and its relation to the differing amount of sunlight hitting the Earth. The three recommended times are:
- In the late fall after you have taught lessons on the properties of light - Mid-February (approximately two months from start date)
- Late April or early May (four months from start date)
Materials:
-stick (an old broomstick works if cut approximately to 1’6” long) secured in a coffee can of stones or something to secure it
-4’ x 8’ sheet of plywood
-waterproof markers for recording (chalk can get smudged, washed away, and ruined) -class list of students divided into groups of four students aka “Shadow Trackers”
Teacher Background:
Our understanding of the passage of time is based upon the motion of the sun or should we
say the “spin cycle” of the Earth. This activity relies on the students remembering and
understanding from their Light Unit activities that the height and direction of a light source
will change the lengths and orientation of the shadow. Using a stick measuring system to
collect data about the size and orientation of the Sun’s shadow the students will begin to learn:
the stick’s shadow pattern will look like a fan of lines, long then short and finally long again,
The shadows will begin to the West of the stick, therefore the light source must be on the East,
The shadows will continue to the East with steeper angles to the edge of the paper,
There is a direct correlation between the angle of the Sun above the horizon and the length of the shadows behind objects – the higher the Sun, the shorter the shadow,
The stick’s shadow is shortest at mid-day.
The construction of a Sun Clock will assist students to apply systematic observation skills in order to have a better sense of “time” as it connects to the movement of the Sun.
Inquiry Activity:
Preparation:
Use the exact outdoor location where you made your observations in the The Sun Moves in the Sky! Activity and secure a 4’ x 8’ sheet of plywood down at your location with one side facing due south. Position the stick near the south side of the wood. If you can’t use
plywood use a 3’ x 6’ piece of butcher-block paper. The plywood is ideal so that you can paint over it and have a permanent location but the paper is ideal if you want to compare several different representations of the shadow’s path.
Activity:
1.
Intro: “Does anyone remember the path of the sun across the sky from The Sun Moves in the Sky! Activity? If you do, who can draw it on the board?” Compare the student’s drawing from memory with your saved and hopefully displayed large
representational drawing from the last activity. If it hasn’t been stated already reinforce that the Sun is moving from left to right or from East to West. Next ask,
“How do shadows change throughout the day?” Ask them to remember back to our light and shadow experiments as we moved the flashlights to make the shadow longer and slanted. Connect their understanding about the position of the flashlight to what we will discover about the position of the Sun. Explain that we will be collecting data outdoors today of how a stick’s shadow changes each hour over the course of a school day. Discuss how we will use a large sheet of wood, a stick and markers to record the shadows and times each hour.
2. Once outside with the students have them sit on the north or top edge to avoid human
shadows on the wood. Ask, “Does anyone know what time it is?” Mark the tip of the
stick’s shadow with a dark marker and label the time and date near the mark. Ask,
“Where is the Sun?” and your answers should include to the left or east. Follow up by asking, “Where is the shadow?” and your answer should include to the right. Using a meter stick ask someone to measure the shadow. Ask, “In an hour who can predict where and how long the stick’s shadow will be? Remember from our drawings of the Sun’s path across the sky and think how that will change the shadow.” Have a student use a stone to mark how high and in what new position they predict the next shadow will fall.
3. Use the same groups of student “Sun Trackers” but this time they’re called “Shadow Trackers”. In one hour send the first group outside to examine where the stick’s shadow is falling. Ask them to examine how the shadow has changed and without looking at the Sun directly describe how the Sun has changed position in the sky.
Have the students mark the shadow’s position with a marker and label the time near the mark on the plywood. Have the group make a prediction for the next hour and place a stone at the predicted position. Additionally, ask the group when they return
“How could the change in the shadow’s size and position be connected to the Sun’s changing position?” If they do not see the connection don’t give them the answer yet but keep asking each group the question after each shadow marking. When you think they get the connection ask, “When do you think the stick’s shadow will be the
shortest? Longest?”
4.
Attempt to go outside with a couple of the “Shadow Trackers” groups throughout the day and discuss how the changes in the shadow’s size are connected to the position of the Sun. Ensure that the students are labeling the times near the shadow marks. If possible, when its time to record the last hour’s shadow mark, have a whole class session out at the observation site. If this is not possible, have a class meeting first thing the following morning. During the whole group session, initially ask, “How did the stick’s shadow change throughout the day?” If the group is not mentioning the role of the Sun’s position in the sky, ask , “Why do the shadows’ position and height change? How is the shadow size and position affected by the position of the Sun?
Does anyone notice a pattern connected to where the shadows fall and their lengths?
Explain why you see a pattern? Is the Sun directly overhead at any time? Why is the shortest shadow around noon? Why does the shortest shadow point North? Briefly discuss how Sun Dials have been used for thousands of years to tell the time based on shadows much like we figured out today.
5. Finally ask the children, “Are our days having more or less light? Is the Sun setting after school earlier or later?” From June 21 until (Summer Solstice) around December 21 (Winter Solstice) the number of sunlight hours decreases and from December 21 to June 21 the number of sunlight hours increases. Now ask, “If we come out in
February will our shadow marks and pattern be the same or different? Explain your
answers.” It is not necessary to go into detail to correct their misconceptions but
acknowledge any answer that mentions that the Sun’s path will get higher in the sky
and will take longer to pass through the sky therefore decreasing the length of the
shadows and changing their hourly positions.
6.
Ideally, you are able to repeat this activity twice before the end of the year. Take a
recording following the same methods in the same location, two months later in winter
and four months later in Spring. Allowing the students to compare and discover the
change in the shadow’s arched path in three different seasons will assist in their ability
to connect the Sun’s altitude or height in the sky, the time of day, the passage of time,
the changing temperature in each season, and the differing amount of sunlight.
Day and Night on the Spinning Globe
A similar activity can be found at
http://hea-www.harvard.edu/ECT/the_book/Chap1/Chapter1.html
In the previous activity, students saw how shadows changed during a day. This activity uses a globe and indoor light
source to create a classroom model showing day and night on a spinning Ear th.
This activity requires a darkened room.
Materials:
Earth globe;
strong focused light source (such as an overhead projector or a slide projector);
golf tees or paper clasps to create shadows on globe;
small figurines;
fun tack or similar material for each group of five if possible;
a picture of Earth taken from space can be helpful.
1. Set up the light source so that it can shine from left to right across the front of the
classroom, without blinding any students with its glare. The students should all be able to see the globe, when held in the beam of light, from about the same direction so that they can see both the illuminated and the dark side of the globe at once.
2. Show students the globe and discuss its use as a model for the Earth. Explain the idea of scale. You can suggest that if the globe were the real Earth, they would be rather large, since they are far larger than the globe! Mark your geographic location on the globe. One of the first concepts that need to be understood is the effect of the Earth’s shape on our ideas of orientation. The globe should be held with the North pole approximately at top;
for today’s purpose the tilt of the axis is irrelevant. Use a figurine to represent a person standing on the Earth near the North pole. Ask students where the person would be looking if they were to look “up.” Where would they look if they looked “down?”
For a person near the pole, these directions align quite well with “up” and “down” in the
classroom, so they should see this. Now position the figure in the southern hemisphere, and repeat the question. There will be some confusion, and you will need to help them see that at any point on Earth, “up” is the direction away from Earth, while “down” is toward the Earth. It is sometimes helpful to show the figuring performing exaggerated “jumps”
off the Earth and “falling” back to Earth, at various locations. Repeat this until students are comfortable with the relative nature of “up” and “down” on a spherical planet.
3. Guide them through questions to the realization that if the globe in your hand were the real Earth, their location in class, off the Earth, would represent outer space. Encourage them to imagine they are out in space, and ask for some differences they would notice.
You should get “no air,” to which you can respond by offering imaginary spacesuits; you may get “no gravity,” to which the response is that in fact gravity does extend to space, but is not felt in an orbiting spacecraft (only bring this complication up when necessary to correct a very pervasive misconception). Eventually, they should come up with “it’s dark,” at which point you can turn off the lights to simulate this situation.
4. Ask then what they might see if they looked down at Earth from their location in space. Is the Earth glowing, like the Sun? The answer is no; the Earth is visible from space only because it is illuminated by Sunlight. Explain that we will use the overhead to represent the Sun today – note that unlike the real Sun this projects light only in a particular
direction – convenient for our purposes today. Turn on the overhead. Half of the globe will seem to “glow” because Sunlight is reflected. Also, people on Earth can now see things around them because the Earth is illuminated. But not all people on earth. Not only half the Earth is illuminated. Place the figurine on the globe at a location where the Sun is “overhead.” Ask them whether the person can see the Sun, whether they create a shadow. Move the figurine to a location on the side of the globe away from the “Sun” and repeat the questions.
5. It seems as though half of the world should be in perpetual darkness and half in perpetual light. Ask them why this is not the case. You should hear that Earth rotates, causing light and dark to alternate at each point. Attach a figurine to the globe at your location with fun tack. Slowly turn the globe from West to East so that the figurine "sees" the Sun rise in the east and set in the west. Have them identify a time the figurine would consider “day” and a time it would consider “night.” Ask them how long they think it takes the Earth to complete a full rotation; the relation to night and day should lead to the right answer.
6.
Now attach a second figurine to another part of the globe. Does the Sun rise earlier or later in this new location? Are the figurines always both in light or both in darkness? Or can one be in light while the other is in darkness? What if the two figurines were on opposite sides of the Earth? Explain the concept of time zones and the reason day at your location is night at other locations on Earth.
7. Attach a golf tee to the globe at your latitude. Again, slowly turn the globe eastward and notice the fan-like shadow pattern that the golf tee casts. Is it similar to the pattern cast by the shadow stick in the previous activities? Note that the shortest shadow points towards the North Pole.
8. Ask, “Have you ever been on a smoothly riding car or train and looked out the window ad
it looked like everything on the outside was moving and you felt like you were standing
still?” Describe now how in the same way because we ride on the spinning earth it seems the Sun and the stars are moving as day and night changes and we fell we are staying still.
Do you feel the earth move? But in fact we are rotating on our axis at close to 1,000 miles per hour. Get the Earth to rotate slowly and ask them “How long it takes for the earth to rotate once?” Ask “when you are on a carousel and you look at the carousel horse you are riding, does it appear to be moving?” After some confusion, they should realize it will appear to be moving up and down only. Ask “if your mother is waiting near the carousel, does she appear to be moving?” They should discover that objects off the carousel appear to be revolving around it, in a direction opposite to the direction in which the carousel spins. If Earth is like a carousel, what should we notice about things that are “off the carousel?” Ask them to name things that are not on Earth; try to get them to realize that the apparent motion of the Sun (and all other objects in space) is precisely of this nature.
9. Attach three golf tees to the globe at various latitudes along the same meridian of
longitude. One should be on the equator, one should approximate your latitude, and one should be near the poles. Ask three students to each observe one of the golf tees. As the globe slowly spins, ask the students to call out their golf tee- "top", "middle", or "bottom"- as they cross the day-night boundary. Also, be sure to observe the midday shadows and to note in which direction they point.
Discussion:
How do we know if we're spinning the globe in the right direction? Where does the Sun rise if we were standing on the globe? Where does it really rise? Set? What if we spun the globe in the other direction? Would this also match our observations? It is only by such
comparisons with observations that we can verify our models. Are the golf tee shadows longer or shorter at the equator? What about at noon, when the Sun is highest in the sky? Is there any shadow at the equator? What about at your latitude? Where do all the shortest shadows point? Does the pattern made by the golf tee reasonably match that made by the shadow stick of the previous activities? Might a spinning earth, then, not be a reasonable model for the passage of day and night? What if the Earth didn't rotate? What if the North Pole were pointed towards the Sun? Where would it be day and night? Would all locations still have both day and night?
Can you think of any other ways to test this model of a spinning Earth? Maybe shadow stick patterns from schools at other latitudes could be compared to yours. Are they consistent with the differences seen on the spinning globe? Have the students observed any complications that our model does not account for? (More on these, such as the tilt of the Earth in the next chapter.)
A useful and fun activity connected to this lesson would be to compare livecam pictures at one time from various locations on Earth, providing a concrete realization of the idea that time of day differs. A game of comparing predictions using the globe with observed conditions can be constructed. Internet Live Cam address:
http://www.rt66.com/~ozone/cam2.htm
Name _______________________________________________
Day and Night
In each of these pictures, we see the Sun on the left and the Earth on the right. We are looking down at the Earth from above the North pole. The arrow shows the direction of Earth’s rotation (to the East).
1. Sunlight will hit one side of the Earth. Color the part of the Earth where it is day in yellow, and the side of the Earth where it is night in black.
2. Draw a stick figure on the Earth where it is noon, so that the person would see the Sun right over their head. Will the person make a shadow?
____________________________________________________________
3. Draw a stick figure of a person for whom it is morning. Will the person make a shadow? Draw an arrow to show the direction of the shadow.
4. Draw a stick figure of a person for whom it is evening. Will the person make a shadow? Draw an arrow to show the direction of the shadow.
5. Draw a stick figure of a person for whom it is midnight. Will the person make a shadow? Draw an arrow to show the direction of the shadow.
Daily Monitoring of Sunrise and Sunset Times
The tilt of the Earth in its orbit about the Sun affects not only the intensity of the solar radiation at a given location, but also the number of daylight hours. These two effects combine to create the weather we usually associate with each season.
In the Northern Hemisphere, the Summer Solstice (longest day, shortest night) occurs around June 21. From June 21 to December 21, the days grow shorter and the nights longer. There are two equinoxes, during which the hours of daylight and night are equal: the Vernal Equinox (around March 21) and the Autumnal Equinox (around September 22). One can observe these changes by recording the times of sunrise and sunset and measuring the length of the day.
Changes in these times are large enough (about a minute a day) to be seen on a graph. The figure below shows such a graph made from sunrise and sunset times at the National Optical Astronomical Observatory located on Kitt Peak near Tucson, Arizona for September to
December. If begun in September and continued through the Winter Solstice (about December 22), the graph should show the gradual decrease in the number of daylight hours (in the
Northern hemisphere) with the minimum at the Winter Solstice. It would be interesting to include either of the equinoxes on the graph as well.
Materials: Large paper from roll (3'X6'); markers; yardstick; pencil; circle labels; paints;
adhesive dots; daily local newspaper.
1. On a large sheet of paper, make a grid on which to plot the sunrise and sunset times.
The time of day should run along the vertical axis, leaving about one inch for every hour.
The date of the observation will be recorded along the horizontal axis.
